In a magnetic resonance device, the body to be examined may be subjected, with the aid of a basic field magnet system, to a relatively high magnetic field, of 3 or 7 Tesla, for example. In addition, a magnetic field gradient is applied with the aid of a gradient system. Radio-frequency excitation signals (e.g., RF signals) are then sent out by suitable antenna devices via a radio frequency transmit system, which is designed to lead to the nuclear spins of specific atoms resonantly excited by this radio-frequency field being flipped by a defined flip angle in relation to the magnetic field lines of the overall magnetic field. This radio-frequency excitation and the resultant flip angle distribution are also referred to as core magnetization. In the relaxation of the nuclear spin, radio-frequency signals (known as magnetic resonance signals) are emitted. The signals are received by suitable receive antennas and then further processed. The desired image data may be reconstructed from the raw data thus acquired. The radio-frequency signals for nuclear spin magnetization may be sent out by a whole-body coil or body coil. A structure for the coil may be a birdcage antenna having a number of transmit rods that are disposed running in parallel to the longitudinal axis around a patient chamber of the tomograph in which the patient is located during the examination. On the front face side, the antenna rods are each connected capacitively with one another in a ring shape. As well as being used for transmission, this antenna may also be used for receiving magnetic resonance signals.
Local coils, applied directly to the body of the patient, are mostly used nowadays to receive the magnetic resonance signals. Such local coils may also be constructed as birdcage antennas. These antennas are constructed with regard to their antenna elements so that the local coils may receive small signals with great sensitivity, which may then be amplified and used as raw data. A birdcage antenna of this type may form a relatively large-surface antenna on or at a very short distance from the body of the examination object or the patient. By placing the local coils close to the body, the best possible signal-to-noise ratio (SNR) may be obtained in the received signal and thus in the diagnostic information.
To create a circular polarization in the birdcage antennas previously (at least) two electric feed points have been provided for such birdcage antennas, which are disposed geometrically so that the feed points lie orthogonally to one another in the circumference of the cylinder. This orthogonality of the feed points demands spatially (at least) two connection points to the birdcage antenna, which in the circumferential direction of a cylindrical carrier tube enclose an angle of 90° to one another.
Previously, a coaxial cable has been connected at each of the feed points as a radio-frequency cable (RF cable), which establishes a direct connection to a radio-frequency final power stage. This is a problem to the extent that the space conditions in the gap between the body coil and the gradient coils of the gradient system are narrow. The fact that the cables lie at an angle of 90° to one another mechanically prevents them being able to be accommodated in a single cylinder sector. For technical reasons, such as the associated attenuation and the required voltage flashover and power density, the coaxial cable may not be made as thin as might be required.
A further problem stems from the fact that the RF cable shares the space with other cables. Many other lines also run to the gradient coils, to the patient interface, to the microphones, monitors, and cameras, etc., in the space between the body coil and the gradient coils. Intersection or even proximity of these lines to the RF cable or cables is both spatially possible with difficulty and also not recommended because of the electric crosstalk.
The power distribution along the antenna rings (also referred to below in the notation as “end rings,” even if the rings are not necessarily present only at the ends) is determining for the creation of the magnetic resonance-relevant B field inside the cylindrical birdcage antenna.
The geometrical orthogonality of the feed points along the cylinder circumference dictates the form and the phase of the sine-wave distribution of the RF currents that flow through the end rings. This current distribution offset alternately at the end face surfaces of the antenna to the end rings, because of the potential difference, is for its part the driving source of the RF currents through the rods. The rod currents are the cause of the effective MR “B1” field and directly influence its spatial orientation, magnitude, and phase through their structure.
The current distribution along the end rings is stationary and resonant, e.g., a “standing wave” is present. In the azimuthal direction on the end rings (along their circumference), each ring has two marked, stationary minima and maxima, of which the position is defined by the type and the position of the feed points. It is resonant in that precisely one full period of a sine or cosine function is completed on the end rings for the basic mode in the azimuthal direction.
The point at which the feed is located geometrically and the type of feed are equally determinant for the position (e.g., spatial phase) of the azimuthal current distribution along the end rings. For this reason, it has been necessary up to now, for the creation of a B1 field with circular polarization, to provide two feed points positioned orthogonally in the circumferential direction, which is controlled electrically phase-offset.
There are however two options for connecting the feed cables to the antenna at its feed points, either symmetrical (also referred to as a connection “along” the antenna element) or asymmetrical in relation to a reference point on the screen between body coil and gradient coil (also referred to as a connection “across” the antenna element).
In symmetrical feeding, this is always applied via a reactive longitudinal component (e.g., a capacitance or an inductance connected in the longitudinal direction of the antenna element) of the antenna. This type of feeding mostly requires a symmetrization in the form of a sheath current filter or/and balun transformer in order to switch from asymmetrical coaxial cable to the symmetrical antenna.
With asymmetrical feeding the feed connection (also called the “feed port”) is located between antenna and a radio-frequency screen that screens the gradient coils from the radio-frequency signals of the body coil (also abbreviated hereafter to GC screen or RF screen). Because both the RF cable and also the feed port are asymmetrical in relation to the reference point “screen,” a forced symmetrization by baluns may not be necessary, but a decoupling of the RF cable by a sheath current filter (e.g., cable trap) might possibly be provided.
The feeding in such cases may either be applied at the end rings or also at the rods, where traditionally end ring feeding is possible.
In recent times (e.g., 2-channel systems with only two feed points), a so-called vertical-horizontal feeding has proved useful. In this case, the feed points of the end ring circumference are not both arranged at the bottom as previously, at an angle of +/−45° from the vertical, but are arranged at the bottom at (e.g., 6 o'clock) and that the side (e.g., at 9 o'clock). This provides that a feed port is attached above the couch. This point however lies in the shoulder area of the patient. Even through the thick support tube wall, the proximity of the patient to the feed points and a sheath current filter associated therewith may have an undesired detuning effect. Therefore, it is more favorable to place the feed ports not in the area of the support tube able to be touched by the patient, but both below the patient couch where possible.
With non-cylindrical (e.g., elliptical or D-shaped) support tubes, which are increasingly being used on account of increased patient comfort, the lateral distance to the gradient coil (e.g., at 3 o'clock and 9 o'clock) is smaller than above and below. In this case, the vertical-horizontal feed mentioned above (e.g., at 6 o'clock and at 9 o'clock) is not possible because of the construction.
The present prior art is thus that the cables have to be routed to two different feed points of the body coil. The feed points lie geometrically orthogonal to one another along the circumference of the cylindrical antenna support tube. At a certain distance away from the feed point outwards along the support tube, the cables may be bent and routed together. However, despite this, the feed points themselves remain geometrically orthogonal.
It would be mechanically more advantageous, but initially electrically not possible, for the feed points to be able to be placed together, so that the cables run in parallel up to the feed point on the antenna. It would be useful to be able to lay the cables through a single “cable duct” at a single point through the circumference of the cylindrical support tube. As a result of the patient couch, a void occurs below the patient that may be used for the routing of the coaxial cables. This may only be done however if the feed points may be placed next to one another or at least in the same circle sector.